Grease and line with parchment paper, a 9 inch round cake pan. Preheat oven at 180.

Whisk together in a large bowl the dry ingredients, flour salt and baking powder. Set aside.

In another bowl mix the cocoa powder with 1/4 cup milk and 1 tbsp of sugar. Mix butter with sugar until creamy. Add vanilla extract.  

With the mixer on low, alternate adding gradually milk and flour mixture until all is well incorporated.

Remove two cups of the batter and mix them with the chocolate mixture. Use one tablespoon for each batter to create the layers of batter on the prepared pan. Begin from the middle of the pan, take one spoon of white batter then over it add a spoon of chocolate batter and repeat these steps until all batter has been used. If you don't have the patience to make these layers just add one batter on top of the other and create swirls with a knife or fork to create the marbled effect.

Bake for 50-60 minutes until the toothpick inserted into the center comes out clean.